The present invention relates generally to the process of making gold-based electrical interconnections for microelectronic devices, including MEMS (Micro-Electro-Mechanical-Systems) devices and IMEMS (Integrated MEMS) devices. Examples of MEMS devices include micro-accelerometers for airbag sensors, micro-mirror arrays for digital light projectors, optical switches for fiber optic systems, and micro-pressure sensors.
Conventional methods for electrically interconnecting a MEMS or IMEMS device to a leadframe, electrode, or other electrical conductor on a package typically use ultrasonically wedge-bonding of an aluminum wire to an aluminized polysilicon bonding pad located on a pre-released MEMS device (see FIG. 1A). For other types of microelectronic devices, including conventional semiconductor IC's, electrical interconnections are made by thermosonically ball-bonding a gold wire to an aluminum bonding pad (see FIG. 1B). IMEMS (Integrated Micro-Electro-Mechanical-Systems) devices combine side-by-side integrated circuits (e.g. CMOS IC's) with MEMS structures on the same substrate (e.g. silicon wafer), also use aluminum wires and aluminum bond pads for electrical interconnections. MOEMS (Micro Optical Electro Mechanical Systems) are MEMS devices that incorporate micro-optical elements, such as micro-mirrors, micro-lenses, and micro fiber optic switches, for example.
Surface micromachined MEMS devices need to be released in order to make them functional (bulk MEMS are already free). The release step generally involves exposing the alternating layers (i.e., levels) of structural polysilicon and sacrificial glass (silicon dioxide) in a multi-level MEMS device to an etchant that includes hydrofluoric acid (HF) or hydrochloric acid (HCl), or a mixture thereof (e.g., a 50/50 mixture). The acid etches away the sacrificial glass layer(s) and releases the delicate polysilicon MEMS structures (e.g., micromachined gears, mirrors, cantilever beams, sliders, pivots, ratchets, etc.). Typically, this release step is performed before the electrical interconnections have been made because the acid etchant will attack and damage the aluminum wires and/or aluminum bonding pads (especially by HCl acid). The acid etchant will also attack any titanium adhesion layer(s), especially by HF acid.
Once the delicate MEMS structures are released and are free to move, they are also unprotected and may be damaged during subsequent packaging steps. These packaging steps can include: sawing, cutting, or scribing the large diameter wafer into individual chips or device dies (singulation or dicing); attaching the device to the package (die attach); wirebonding; flip-chip solder bumping; thin or thick-film metallization; plating; pre-seal inspection; sealing of hermetic or dust protection cover lids; windowing; package sealing; trim; marking; final test; shipping; storage; and installation (e.g., soldering or plugging into another assembly or board). Potential risks to the delicate released MEMS structures include: electrostatic effects, dust, moisture, contamination, handling stresses, shock, and thermal effects. The term “die singulation” includes laser cutting, sawing, scribing, precision breaking, and water jet cutting.
After the MEMS structures have been released, a thin layer(s) of anti-stiction coating or lubricant, e.g., self-assembling monolayers (SAM coating), may be applied to reduce friction and enhance performance. However, presence of these coatings on the surface of the polysilicon bonding pad can act as an organic contaminant and interfere with making a good electrical interconnection.
The reliability and yield of packaged MEMS or IMEMS devices could be significantly improved if the acid etch release step could be performed after as many of the potentially harmful packaging steps had been completed as possible. In particular, the acid etch release step could be performed after the electrical interconnections have been made. This would be possible if the electrical interconnections were either: (1) isolated from the acid etchant, and/or (2) made from materials (such as gold) that are naturally resistant or impervious to attack by HCl or HF acid. Therefore, a release resistant electrical interconnection for MEMS or IMEMS devices preferably would not use aluminum wires, aluminum or aluminized bond pads, or titanium adhesion layers. Here, we define “aluminum bond pads” as including (1) a bond pad made of aluminum (or Al-alloy) bonded to a silicon substrate, or (2) an aluminized coating (metallized aluminum) on top of a polysilicon bonding pad. We also define MEMS devices to generically include IMEMS (Integrated MEMS) and MOEMS (Micro Optical Electrical Mechanical Systems) devices.
U.S. Pat. No. 6,500,760 “Gold-Based Electrical Interconnections for Microelectronic Devices”, which is incorporated by reference, describes methods of fabricating release-resistant electrical interconnections for microelectronic devices, including MEMS and IMEMS devices.
Against this background, the present invention was developed.